Light Detection and Ranging (lidar) systems are used in numerous areas of practical interest to make remote measurements. In lidar systems, a light beam is sent to a target and a detection system is used to extract information about the target.
Heightened concerns in recent years about potential use of airborne chemical and biological agents to cause harm has increased the urgency of finding methods to remotely detect and locate such agents. It has generally been found difficult to use remote sensing methods to a) detect threat species and b) reliably discriminate the threat species from other species that may also be present or classify the threat species. In the case of biological agents like anthrax it is in principle possible to detect their presence by simply collecting scattered light from the particles. However, simple light scattering measurements often cannot tell the difference between types of aerosols. For example, the scattered light “signature” of an anthrax particle is similar to the scattered light signature of common dust.
To enhance stand-off biological agent discrimination and/or classification, other laser based remote sensing techniques have been developed, in particular Laser Induced Fluorescence or LIF. In LIF, a short wavelength (typically in the UV or visible spectral range) laser illuminates the particles, the light is absorbed and subsequently re-emitted at a different (longer) wavelength. By detecting the longer wavelength emission one may infer that a biological aerosol is present (since inorganic materials tend not to fluoresce). However, there is frequently little in the fluorescence signature that permits one to distinguish one biological species from another.
Polarization and wavelength normalized depolarization ratio lidars have been used to discriminate stratospheric ice from water and bacillus globigii (Bg) from dust, pollens, and smokes, respectively. Discrimination based on depolarization measurements is primarily dependent on the degree of non-sphericity of particles. However, as the types of biological aerosol species expand, and the various methods of biological species preparation and dissemination methods are assessed, more robust measurement techniques are needed.
What is needed are systems and methods capable of detecting and discriminating and/or classifying hazardous biological agents to enable suitable action to taken when a threat species is found. In addition, systems and methods should desirably permit detection at several kilometers. In addition, systems and methods should be capable of day or night operation and be operationally eye-safe.